This research will further integrate the fields of behavioral endocrinology and biological rhythms. Experiments designed for circadian and annual rhythm analysis will benefit from the application of established methods practiced by researchers studying hormones and behavior. Projects include 1) examination of sexual differentiation of rhythm response to estrogen and of sex differences in the control of wheelrunning rhythms; 2) assessment of neural substrates mediating responsiveness of circadian rhythms to estradiol using hormone implant and lesion techniques; 3) study of the interaction of photoperiod and internal rhythm mediators (e.g., hormones and brain) with respect to controlling hamster reproductive rhythms.